Progenitor cells are undifferentiated, non-specialized cells that give rise to particular cell lineages. For example, muscle progenitor cells (myoblasts) give rise to muscle cells, neuronal progenitor cells (neuroblasts) differentiate to produce nerve cells, and hematopoetic progenitor cells are the precursors of cells found in blood and lymph. Understanding the molecular controls of cellular differentiation from progenitor cells to mature cells has important implications in research about, and therapy for, many different diseases associated with tissue degeneration and/or atrophy, including muscular dystrophy, Alzheimer's disease, and anemia.
For example, myoblasts differentiate into multinucleated myotubes, which develop into mature muscle fibers (myocytes). This differentiation occurs naturally in vertebrate mammals, but also may be accomplished artificially. During transition from myoblasts to myotubes, two muscle-restricted proteins, MyoD and Myf5 (which are functionally inert in myoblasts) trigger a cascade of cellular events leading to the irreversible cell cycle arrest, the expression of other myogenic regulatory factors (MRFs), and the transcription of a number of differentiation-specific genes encoding structural and contractile proteins. See, e.g., Lassar, A. and A. Münsterberg, Current Opinion in Cell Biology 6:432–42 (1994); Molkentin, J. D. and E. N. Olson, Current Opinion in Genetics & Development 6:445–53 (1996).